Performing physiological functions, muscular tissue changes its characteristics. The study of these changes, through assessment of impedance modification, has proven useful both in research and clinical fields. Electrical impedance spectroscopy is a relatively new technique and very promising in which the tissue impedance is calculated at different frequency values. This work aimed to study the muscular tissue changes in correspondence of different subjects' characteristics and different muscle conditions, rest, contraction and phase after contraction, with a concise index related to the position of Nyquist plots in the complex plane. The study was carried out by applying EIS to the group of the forearm flexor muscles in vivo in 33 subjects. Obtained results show that Nyquist plots are arranged in different ways in correspondence of various measurement conditions and subjects' characteristics.